1. Field of the Invention
The present invention relates to an amino compound for an organic light-emitting device and an organic light-emitting device including the same. More specifically, the present invention relates to an organic light-emitting device that emits light by applying an electric field to a thin film made of an organic compound.
2. Description of the Related Art
In organic light-emitting devices, a thin film containing a fluorescent organic compound or a phosphorescent organic compound is sandwiched between an anode and a cathode, and holes and electrons are injected from the electrodes to the thin film.
Accordingly, excitons of the fluorescent compound or the phosphorescent compound are generated, and the device uses light emitted when these excitons return to the ground state.
Recently, a marked development has been achieved in such an organic light-emitting device. According to the features of the organic light-emitting device, a high luminance can be produced with a low applied voltage, a diverse emission wavelength can be provided, a high-speed response can be realized, and the thickness and the weight of the device can be reduced, thus suggesting the possibility of a wide variety of applications.
In the current situation, however, it is necessary to yield an optical output with a higher luminance or a higher conversion efficiency. In addition, conventional organic light-emitting devices still have many problems in terms of durability, for example, a change over time due to long-term use, and degradation caused by an atmospheric gas containing oxygen, moisture, or the like.
When applying an organic light-emitting device to a full-color display or the like, light emissions of blue, green, and red with satisfactory color purity are also required. However, conventional devices have not yet achieved satisfactory properties.
Japanese Patent Laid-Open Nos. 11-144875, 2003-48868, and 2005-119994 disclose examples of a material containing a fluorene compound and an organic light-emitting device. However, Japanese Patent Laid-Open No. 11-144875 (JP'875) discloses, as an aryl-substituted aryl group that is substituted at the amino position of a difluorenylamine, only a phenyl group which is substituted at the 4th position with a phenyl group. JP'875 does not disclose a compound in which fluorenyl groups are substituted at the 3rd position and the 5th position, or at the 2nd position and the 6th position of the phenyl group.
A compound as in JP'875 in which a phenyl group of N,N-difluorenyl-N-phenylamine is substituted with a phenyl group has a molecular weight which is less than that of a compound in which the phenyl group of N,N-difluorenyl-N-phenylamine is substituted with a fluorenyl group. The former compound as disclosed in JP'875 tends to have a lower glass transition temperature (Tg) than the latter compound. Therefore, it is believed that the latter compound, which is substituted with a fluorenyl group, provides a more dimensionally stable film having better properties and is more suitable for an organic light-emitting device. Furthermore, a fluorenyl group has a conjugated plane larger than that of a phenyl group, and, thus, the overlap of orbital electrons is increased. Accordingly, charge-transporting capacity is also increased, and a high electron or hole mobility can be expected.
Accordingly, it is believed that the introduction of a fluorenyl group provides a film with high Tg and high charge mobility.
In general, as orbital conjugation extends, band gap becomes narrower, and, conjugation becomes narrower, the band gap widens. Furthermore, in general, when aromatic rings are bonded at the ortho-position or the para-position, conjugation is extended. On the other hand, when aromatic rings are bonded at the meta-position, conjugation is not extended.
In the compound in which the 4th position of a phenyl group of N,N-difluorenyl-N-phenylamine is substituted with a phenyl group as in JP'875, conjugation extends from the phenyl group substituted at the 4th position to the two fluorenyl groups via the phenyl group and the amine, thereby narrowing the band gap. In contrast, when two fluorenyl groups are bonded at the 3rd position and the 5th position, or at the 2nd position and the 6th position, i.e., at the meta positions, conjugation is not extended, thereby providing a wide band gap.
In general, when a compound is used as a hole injection/transporting material and the band gap of the material is narrow, the band gap of a luminescent layer also tends to be narrow. In this embodiment, it is difficult to select a host compound in a device requiring a relatively wide band gap, for example, blue fluorescence or phosphorescence. When a compound is used as a host material, the material must have a band gap wider than that of a guest material. In particular, in phosphorescent materials, the energy level at the lowest excited triplet state (T1) must be higher than the T1 of the guest. Accordingly, the phosphorescent materials require a wide band gap.